Role of charge-transfer for excitons and their delocalization in van der Waals magnetic semiconductors*

In semiconductors, the coupling of excitons in the optical regime (∼1-4 eV) to the spin degrees of freedom is the basis for the manipulation of magnetic properties using light. For this reason, intense interest has arisen from the observation of sharp optical excitations within the magnetic phase of Ni-based van der Waals (vdW) semiconductors (e.g. NiPS₃). However, the microscopic description of these excitations remains an essential outstanding question, including their momentum dispersion, relationship to magnetic order, and the role of metal-ligand charge-transfer processes. Here, we uncover each of these aspects in the nickel dihalides (NiX₂, X = Cl, Br, I) using Ni-L₃ resonant inelastic x-ray scattering (RIXS). We detect sharp excitons in all compounds and unambiguously assign them to spin-singlet multiplets of octahedrally-coordinated Ni²⁺ (i.e. dd excitations). RIXS directly reveals that these excitons disperse. Their dispersion is independent of temperature and magnetic phase and distinct from the spin excitation dispersion which is simultaneously recorded at lower energies. Our data therefore suggest a ligand-mediated multiplet delocalization which is tuned by the charge transfer gap regardless of long-range magnetic order as highlighted by our phenomenological model. These results thus resolve the microscopic nature of the excitons, their relationship to magnetism, and establish them as a general phenomenon for the physics of charge transfer insulators and semiconductors.